HANDBOOK
OF
CHEMICAL ENGINEERING
SECTION I
POWER GENERATION AND TRANSMISSION
BY W. D.
Definitions. — The foot-pound is the unit of work. One foot-pound is the work done in raising a 1-lb. weight a distance of 1 ft. vertically against gravity. The heat unit (British thermal unit, B.t.u.) is also a unit of work. It is Hso of that amount of heat necessary to raise the temperature of 1 Ib. of water from the freezing point to the boiling point. One large calorie = 2.2046 X % = 3.968 B.t.u. One British thermal unit = 778 ft.-lb. The horsepower is the unit rate of doing work. One horsepower = 33,000 ft.-lb. per minute = 550 ft.-lb. per second = 2,545 B.t.u. per hour = 42.42 B.t.u. per minute. The horsepower-hour is a measure of work done. One horsepower-hour = 60 X 33,000 = 1,980,000 'ft.-lb. or 2,545 B.t.u. Concurrent units are the kilowatt and kilowatt-hour. One kilowatt = 1.34 hp. One kilowatt-hour = 2,650,000 ft.-lb. = 3,410 B.t.u. Registering instruments measure kw.-hr. If the work done in 5 hr. is 50,000 kw.-hr., the average load during that period was 10,000 kw. or 13,400 hp.
Load may be expressed in horsepower or kilowatts; or, for boilers and process work, in pounds of steam per hour; or as measured by consumption (pounds of coal per hour). The load curve is a graph of load against time. Figure 1 gives load at 2-hr. intervals for 1 day. Each ordinate is the quotient of kilowatt-hours registered in 2-hr, by 2. An indicating instrument would show fluctuations above and below the line. The base may be successive days in a month or year.
Average load over any time, t hr., = kilowatt-hours during that time -J- t. In Fig. 1 it is 1,219 kw. Maximum load may be taken as the highest maximum shown by an indicating instrument, or (more usually) the highest average maintained during a stated period. In Fig. 1 the maximum or peak load maintained for a 2-hr, period is 1,870 kw. Load factor LF — average load -f- maximum load, for the stated periods. In Fig. 1, LF = 1,219 -J- 1,870 = 0.65. It is always less than unity. The annual LF will be high in plants operating on a 24-hr, schedule. For electric central stations, it is usually between 0.30 and 0.40. The LF while running, in machine shops, may be as high as 0.50. In few industries is it over 0.60. Rated capacity of any machine is generally that capacity or output at which it is most efficient. If the rated capacity of this plant in Fig. 1 is 1,800 kw. it will operate at less than maximum efficiency at all loads other than 1,800 kw. Overload capacity = (maximum safe load — rated capacity) -r- rated capacity. If the plant can carry a maximum safe load of 2,400 kw. its overload capacity is (2,400 - 1,800) •*- 1,800 = 0.333. Overload = (actual load — rated capacity) -r- rated capacity. For this plant, between 8 and 10 a.m. (Fig. 1) the overload is (1,870 - 1,800) -5- 1,800 = 0.04 (4 per cent). Use factor UF or capacity factor = average load -5- rated capacity. Here UF = 1,219 •*• 1,800 = 0.68. Reserve capacity = (maximum safe load — maximum actual load) •*- maximum actual load. If there were no fluctuations from the line of Fig. 1, this would be (2,400 — 1,870) -r- 1,870 = 0.28. Reserve capacity gives a margin of apparatus to offset possible breakdowns. Diversity factor DF = sum of maximum demands of units com-1 Vice-Pres., Technical Advisory Corporation, 132 Nassau St., New York.
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